Vacuum capacitor with solid dielectric

ABSTRACT

A unique combination of vacuum component techniques to a solid dielectric capacitor. The vacuum-type enclosure includes at least one generally cylindrical body shell member of insulating material and one or more metallic cylindrical sections attached thereto. A control shaft operating through a vacuum-type bellows moves an internal cylindrical member encased in an insulating material sleeve of polytetrafluoroethylene to provide variable capacitive engagement within at least one of the outer metallic shell members. The bearing arrangements common in vacuum variable capacitors are eliminated, because the fluorocarbon insulating dielectric material preserves the spacing and coaxial relationship of the inner and outer shells and provides a sliding surface of very low friction. Advantage is taken of the desirable dielectric strength of the fluorocarbon dielectric material without the destructive effects of corona in air. An alternative planar element embodiment of the invention using disc plates and a spring disc variable plate is also shown. In that embodiment the dielectric is also a planar element.

United States Patent Lindsay [54] VACUUM CAPACITOR WITH SOLID DIELECTRIC[72] Inventor: Wesley N. Lindsay, San Jose, Calif.

[73] Assignee: International Telephone and Telegraph Corporation, NewYork, NY. [22] Filed: Aug. 10, 1970 I21] Appl, No.: 62,324

Dummer variable Capacitors and Trimmers Pitrnan & Sons London 1963 pp.84- 85 [451 Feb.1,1972

Primary Examiner-E. A. Goldberg Att0rney-C. Cornell Remsen, Jr., WalterJ. Baum, Paul W. Hemminger, Charles L. Johnson, Jr. and Thomas E.Kristofferson [5 7] ABSTRACT A unique combination of vacuum componenttechniques to a solid dielectric capacitor. The vacuum-type enclosureincludes at least one generally cylindrical body shell member ofinsulating material and one or more metallic cylindrical sectionsattached thereto. A control shaft operating through a vacuum-typebellows moves an internal cylindrical member encased in an insulatingmaterial sleeve of polytetrafluoroethylene to provide variablecapacitive engagement within at least one of the outer metallic shellmembers. The bearing arrangements common in vacuum variable capacitorsare eliminated, because the fluorocarbon insulating dielectric materialpreserves the spacing and coaxial relationship of the inner and outershells and provides a sliding surface of very low friction. Advantage istaken of the desirable dielectric strength of the fluorocarbondielectric material without the destructive effects of corona in air. Analternative planar element embodiment of the invention using disc platesand a spring disc variable plate is also shown. In that embodiment thedielectric is also a planar element.

l QClaims, 5 Drawing Figures VACUUM CAPACITOR WITI-I SOLID DIELECTRICBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to variable vacuum capacitors.

2. Description of the Prior Art Vacuum capacitors in a variety ofconfigurations have long been extant in this art. The most common typesinvolve movable and fixed sets of interleaving cylindrical plates in anevacuated envelope. The evacuated space, with its unity dielectricconstant, predetermines the capacitance/size relationship for givenvoltage and power ratings in such devices. Moreover, rather expensiveand carefully aligned bearings are required.

It is the foregoing disadvantages which generate the need for thepresent invention.

SUMMARY lt may be said that the general objective of the presentinvention was to provide a solid dielectric capacitor exhibitingsubstantially all of the well-known advantages of vacuum capacitors, andin addition, affording higher capacitance per unit volume than possiblewith prior art vacuum capacitors.

The unique advantages of the present invention are obtained through theuse of a solid dielectric having a substantially higher dielectricconstant than that of air or an evacuated space. The problem ofselecting a suitable dielectric material for such capacitor asenvisioned in the present invention, is not at all responsive to thewell-known prior art capacitor dielectric criteria because of themultiple functions required of the dielectric layer. Although a greatvariety of dielectric materials exhibiting low losses in specificfrequency ranges and relatively high dielectric constants have beenemployed in the past, comparatively few of such dielectric materials areadapted electrically and physically for inclusion in the presentinvention. The physical criteria for a suitable dielectric material foruse in capacitors according to the present invention include low vaporpressure, high melting point and a lubricatory surface characteristic.

In capacitors constructed in accordance with the present invention,fluorocarbon polymers have been found to be very satisfactory. Inparticular, fluorocarbon polymers with low vapor pressure and highmelting point are preferred. Such a material is polytetrafluorethylene,commercially known as Teflon."

The outgassing process in the manufacture of vacuum components involvesthe application of heat as a processing step. Accordingly, the physicalproperty requirements for the required dielectric material will beunderstood. The aforementioned polytetrafluoroethylene material has beenfound to exhibit the appropriate outgassing compatibility in that it hasa low vapor pressure and high melting point. It also exhibits thedesired lubricatory surface quality desired and is electricallysatisfactory.

This description illustrates several embodiments in whichthe propertiesof the Teflon dielectric material are exploited in several ways. It isknown that Teflon deteriorates rapidly in the presence of corona when itis applied as a high voltage dielectric in the atmosphere. Operation asa high voltage dielectric in an evacuated vessel eliminates this problemand makes it possible to exploit the other properties of the material.

In the several embodiments shown, it will be realized that the overallmechanical structure is substantially simplified as compared to theprior art because the movable plate, or plates, are actually supportedby the Teflon dielectric as they are caused to slide over itslubricatory surface. Expensive bearing structures are thereby eliminatedand a further contribution is made to the reduction in size and weight.

Capacitors constructed'in accordance with the present invention areparticularly adaptable for use as high voltage trimmers or neutralizingcapacitors in such applications as; radio frequency amplificrs.;oscillators, radio frequency power couplers and antenna matching units,to mention just a few possible applications.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional drawing of atubular plate version of the present invention with two electricallyseparate fixed electrodes and a single movable electrode resembling adifferential split-stator) capacitor.

FIG. 2.,is a sectional drawing of a high voltage, single fixed electrodeembodiment of the present invention.

FIG. 3 is a modification of the embodiment of FIG. 2 for use where thedegree of external flashover insulation required is lower.

FIG. 4is a planar element version of the present invention, also shownin section, employing a resilient deformable movable electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For convenience in thisdescription, the aforementioned polytetrafluoroethylene will be referredto as Teflon. A reference listing some properties of that material isthe Handbook of Chemistry and Physics" (49th Edition, published byChemical Rubber Company of Cleveland, Ohio). In that handbook, underProperties of Commercial Plastics, materialsof the chemical classpolytetrafluoroethylene are identified and some of their properties ofinterest are listed. The material is otherwise well known and has beenwidely used in electrical and nonelectrical applications.

Concerning the selection of appropriate materials for the otherillustrated parts of devices in accordance with the invention, it ispointed out that in the vacuum component prior art, much is known aboutappropriate constructional materials. Also the prior art teaches methodsof constructing and sealing vacuum type enclosures which include ceramicor glass insulator sections used with metallic enclosure members. It isalso well understood that the process step in which the enclosure isevacuated requires an access opening, generally known in this art as atubulation, which is sealed after the evacuation process is completed.Equivalent measures are required in connection with the construction ofthe present invention however, they are omitted from the drawingssubmitted with this specification for clarity since they are not a novelpart of the present invention.

Referring now to FIG. I, a differential split-stator arrangement isillustrated. The vacuum enclosure includes the two stator or fixedelectrodes, 1 and 4, which are separated by an insulating tubularsection 2 sealed to 1 and 4 at 7 and 5, respectively. Another insulator3 is sealed to the metallic electrode 4 at 6 and to an endplate 9 at 8.An inside metallic tubular section 15 serves as the rotor, or movable,electrode. The closed end of the element 15 is illustrated at 15a, andit is within the inside of this closed end that a leadscrew I1, isattached. The endplate 9 has a center clearance hole through which theleadscrew 11 is allowed to project to the right of the figure, asindicated. The seals 5, 6, 7 and 8, are, of course, vacuum type seals.The same can be said of sealing of the bellows 12 to the inside of themovable electrode (within 15a) and the seal of the said bellows to theendplate 9. Accordingly, the evacuated space is that around the outsideperimeter of the bellows l2 and includes the space substantially filledby the Teflon sleeve 14 which surrounds the movable electrode 15. The.said sleeve 14 preferably includes a complete wraparound at Ma aroundthe end 15a of the said movable electrode without this Teflon wraparound14a, the movable structure obviously cannot be permitted to closelyapproach the inside end of the member 1.

For the sake of structural completeness, an adjustment knob 10 isillustrated. This knob would have internal threads to mate with theexternal threads of the leadscrew 11. In view of the evacuated space asdescribed, including the volume at 13, it will be apparent that theexternal air pressure will act to tend to extend the bellows 12, thatis, external air pressure will tend to mesh movable electrode 15 withthe fixed electrode 1 to the maximum extent. Accordingly, the adjustmentknob 10 is thereby mechanically biased against the endplate 9. Thiseffect is helpful in eliminating lost motion" in the thread engagementbetween and 11. Obviously, counterclockwise rotation of 10 would causemovement of the screw 11 axially to the right or the left, dependingupon the sense of the thread pitches. Counterclockwise rotation would,of course, produce the converse.

If the capacitor of FIG. 1 were mounted at the endplate 9, such that aground existed there, the movable electrode 15 would thereby begrounded. If, however, a mounting was otherwise provided so that theplate 9 was floating electrically, it would obviously be possible toprovide a separate electrical contact thereto and to construct theadjustment knob 10 of insulating material, possibly with a cap over theprotruding end of the leadscrew 11.

The fixed electrodes 1 and 4 will be seen to provide their own externalelectrical access or contact surface. The axial lengths of the elements1, 4 and 15 arearbitrarily illustrated in FIG. 1, and obviously thesecould be varied in proportion to produce any desired differentialcapacitance relationship among the said fixed and movable electrodes.

, Referring now to FIG. 2, an embodiment of the present invention isillustrated in a single ended form. The ceramic tube 17 is relativelylong in comparison with the total length of the device, an expedientwhich provides for high voltage operation of the capacitor.

Most of the description taken in connection with FIG. 1 is applicable toFIG. 2, the differences being substantially obvious from inspection ofthe figures, once the operation and structural details of FIG. 1 areunderstood.

A leadscrew 22 and adjustment knob 23 operate substantially identical.to their counterparts l0 and 11, respectively, from FIG. 1. Thebellows'lS, moreover, is sealed and functions the same as counterpartmetal bellows 12 from FIG. 1. The metal tube is electrically anextension with the ceramic insulating tube 17 of the element 16, thelatter being the externally connectable portion of the fixed electrode.The inside metal tube 21 forms the movable electrode'and is separated bythe Teflon sleeve 19 which also completely engulfs the end (left asillustrated) of 21.

Referring nowto FIG. 3, a lower voltage version of the single endedcapacitor of FIG. 2, is illustrated. Except for the longer fixedelectrode 26, the shorter insulating ceramic tube 24 and the tubular endclosure 25, in lieu of the endplate, such as 9 of FIG. 1, the structureof FIG. 3 does not differ substantially from that of FIG. 2. The movableelectrode 27 is surrounded by essentially the same type of Teflon sleeve28, as

' found at 19 in FIG. 2. In FIG. 2, electrical connection to the fixedelectrode 26 is afforded directly at its outside surface, whereas themovable electrode 27 may be contacted along the metallicmember which isin contact with the movable electrode through the leadscrew and bellows.In view of the shorter relative length of the ceramic insulating tubularmember 24, the embodiment of FIG. 3 may be expected to be applicable tolower voltage applications than that of FIG. 2.

Referring now to FIGS. 4 and 5, an embodiment is illustrated in whichthe fixed electrode 30 is substantially planar or disclike, and ismounted on a metallic mounting plate 29. Such a device is adaptedto highpower radio frequency bypassing applications, or the like. Of course itis not necessary that the fixed electrode 30 be grounded to a mountingplate, an insulating mounting about the ceramic envelope 34 (forexample) being easily substituted.

The familiar shaft or leadscrew 35 with threaded portion 39 andcooperating adjustment nut 38 will be recognized. Since the amount ofshaft movement in a capacitor of this design is relatively small, thebellows 36 is correspondingly short and is sealed to the shaft 39 at apoint within the evacuated chamber, as illustrated, The fixed electrode30 is, of course, also sealed to the bottom perimeter of the ceramicenvelope 34, and a corresponding Teflon dielectric layer in the form ofa disc is shown at 33. Over this, a flexible electrode 32, constructedof a resilient metallic material, for example, such as frequently usedfor finger stock. This movable electrode 32 is illustrated in a flatview in FIG. 5. The movable or flexible eleclowered. As the perimeter ofthe part32 increases, due to.

downward adjustment pressure from 31, the wedge-shaped relief openings,shown in FIG. 5, tend to expand somewhat. The number of relief cutsdepicted in FIG. 5 is not necessarily limiting and is arbitrarilyillustrated. Many more and smaller width radial cuts could have beenprovided, giving the part the appearance of a large number of radiallyprojecting close spaced fingers on a central disc. As in the otherembodiments discussed, the effect of external air pressureis such as totend to cause the capacitor to assume its fully meshed position, theadjustment nut resisting this tendency and enforcing a predeterminedsetting. 3

It will be noted that the backing plate is illustrated in FIG. 4 asbeing slightly dished upwardly around its perimeter. This perimeter iscontained within the inside diameter of the ceramic envelope 34, thisdesign serving'to align the shaft 35 laterally. The perimeter of thepart 32 operates against the lubricatory surface of the Teflondielectric layer, and accordingly, it will be realized that no bearingstructure, as commonly encountered in prior art devices, is required inthe embodiment of FIG. 4, or, for that matter, in the embodiments ofFIGS. 1 through 3. In all cases, the sliding of the movable electrode,or plate, of the capacitor, is accomplished against the lubricatorysurface of the Teflon dielectric layer. In the embodiments of FIGS. 1through 3, the motion of the movable electrode is entirely axialtranslation; however, the deformation of part 32 in the FIG.'5 is partof the'motion in that embodiment. Nevertheless, the advantage of thelubricatory Teflon surface is employed in all embodiments illustrated.This low slidingfriction characteristic of the Teflon applies where itslides against the ceramic body shell members in the piston typeembodiments of FIGS. 1 and 3.

The adjustment nut- 38 bears against the closure endplate 37. Thebellows 36, being conductively sealed to the metallic rod 35, providesan external contact for the .movable plate at the said end plate 37.

The said piston type embodiments provide substantially linearcapacitance variation as a function of adjustment knob rotation untilthe added effectof the approach of. the tubular electrode closed endsbecomes significant (i.e., at nearly fully meshed positions). l

The embodiment of FIG. 1 has the advantage of eliminating bellows andshaft current, provided external connections are consistent with thatobjective. Accordingly, the device of FIG. 1 has a relatively lowequivalent circuit inductance and a higher Q due to reduced current pathlosses (mainly due to bellows resistance).

It has been determined that Teflon can be baked out" to a degreeconsistent with vacuum levels of 10* Torr.

A capacitor constructed in accordance with the present invention can beexpected to withstand peak voltages up to 28 kilovolts and have at leasta 20 kilovolt stable working rating with 20-mil Teflon dielectric. Inair, this performance would be impossible as the Teflon yielded to thedamaging effects of corona.

It will be apparent that features of each embodiment could beinterchanged with exercise of ordinary mechanical and electrical designskills. Although ceramic body insulator sections are illustrated, glasssections could serve the same purpose. Both ceramic and glass bodyinsulator sections have been widely used in prior art vacuum components.

Although Teflon has been cited as the preferred fluorocarbon material,other dielectric material with equivalent lubricity, dielectricstrength, and low vapor pressure can,'of course, be used. Thesignificant parameters for the dielectric are:

coefficient of friction in vacuum less than 0.2, dielectric strengthgreater than 1,000 volts per mil, and vapor pressure less than 10* Torr.

Various other modifications and variations are possible within the scopeof the present invention. Accordingly, it is not intended that the scopeof the claims should be limited by this description or the drawings, asthese are intended to be illustrative and typical only.

What is claimed is:

l. A solid dielectric vacuum capacitor comprising:

a hermetically sealed evacuated enclosure; a movable and at least onefixed capacitor electrodes each having at least one surface within saidsealed enclosure;

' an elongated mechanical control member connected to said movableelectrode to control said movable electrode in a predetermined mode ofmotion which is at least partially sliding motion;

an extendable bellows sealed at one end to said enclosure and along thelength of said elongated control member to permit said predeterminedmode of motion of said movable electrode within said sealed enclosure;

means for providing separate external electrical connections for saidfixed and movable electrodes, including electrical insulation meansforming a part of said enclosure for providing electrical separation ofsaid electrodes;

and a dielectric layer emplaced between said movable and fixedelectrode, said dielectric being composed of insulating materialexhibiting a lubricatory surface in vacuum, relatively high dielectricstrength and vapor pressure less than l- Torr.

2. The invention set forth in claim 1 in which said dielectric layer isdefined as being composed of a fluorocarbon polymer having a highmelting point.

3. The invention set forth in claim 1 in which said dielectric layer iscomposed of polytetrafluoroethylene.

4. Apparatus according to claim 2 in which said enclosure is elongatedand generally tubular in shape, said electrical insulation means forminga part of said enclosure comprises a generally cylindrical shell ofinsulating material, said fixed capacitor electrode comprises agenerally tubular metallic extension of said insulation means having aclosed end, said movable electrode is a coaxial generally tubular memberfitting within said fixed electrode and said dielectric layer isemplaced between the inside surface of said fixed electrode and theoutside surface of said movable electrode.

5. Apparatus according to claim 4 in which said dielectric layer isfixed to the outer periphery of said movable electrode, thereby to forma lubricatory interface with said fixed electrode.

6. Apparatus according to claim 5 including two of said fixed electrodesin axial series separated by said cylindrical shell of insulatingmaterial, and in which said movable electrode is axially positionable indifferential capacitance relationship with respect to said two fixedelectrodes.

7. The invention set forth in claim 2 in which said fixed electrode is asubstantially planar surface normal to the axial centerline of saidenclosure, said movable electrode is a generally disc shape member ofresilient metal predisposed to assume a generally conical shape in theabsence of axially applied force, said dielectric layer is a planarsheet emplaced between said fixed and movable electrodes, and saidmechanical control member is arranged to exert a force tending toflatten said movable electrode as its peripheral edges slide radiallyoutward thereby to correspondingly control the capacitance between saidelectrodes.

8. The invention set forth in claim 5 in which said movable electrode isa generally tubular closed-endmember and said dielectric layersubstantially encases said movable electrode, whereby the fully meshedcapacitance between said fixed and movable electrodes is proportional tothe surface area of said movable electrode in juxtaposition with saidfixed electrode, including the area of said closed end.

9. A solid dielectric variable vacuum capacitor comprising: at least onegenerally tubular metallic outer member forming a first capacitor plate;

a generally tubular metallic inner member of outside diameter smallerthan the inside diameter of said outer member;

a generally tubular fluorocarbon polymer dielectric member of inner andouter diameters differing from said inner member outside diameter andsaid outer member inside diameter, respectively, by substantially onlysliding fit allowances, said inner member being axially slidablysupported within said outer member by said dielectric member withoutadditional bearing means;

a conductive mechanical control member connected for providing axialsliding movement of said inner tubular member to vary the capacitancebetween said inner and outer members by varying the axial insertion ofsaid inner member within said outer member;

an evacuated enclosure including said outer member as a part thereof,said enclosure providing evacuated space at least enclosing the adjacentsurfaces of said inner and outer tubular members and said dielectricmember, said enclosure also including an extendable metallic bellowssealed between said mechanical member and an end of said enclosure topermit axial movement of said mechanical member within said enclosure;

and insulating means for providing an electrically insulated mountingfor said mechanical control member externally adjacent the connection ofsaid mechanical control member and said bellows, thereby to electricallyisolate said inner and outer members, said inner member making externalelectrical connection through said bellows and said mechanical member.

10. Apparatus according to claim 9 in which said fluorocarbon polymer ispolytetrafluoroethylene.

1. A solid dielectric vacuum capacitor comprising: a hermetically sealedevacuated enclosure; a movable and at least one fixed capacitorelectrodes each having at least one surface within said sealedenclosure; an elongated mechanical control member connected to saidmovable electrode to control said movable electrode in a predeterminedmode of motion which is at least partially sliding motion; an extendablebellows sealed at one end to said enclosure and along the length of saidelongated control member to permit said predetermined mode of motion ofsaid movable electrode within said sealed enclosure; means for providingseparate external electrical connections for said fixed and movableelectrodes, including electrical Insulation means forming a part of saidenclosure for providing electrical separation of said electrodes; and adielectric layer emplaced between said movable and fixed electrode, saiddielectric being composed of insulating material exhibiting alubricatory surface in vacuum, relatively high dielectric strength andvapor pressure less than 10 4 Torr.
 2. The invention set forth in claim1 in which said dielectric layer is defined as being composed of afluorocarbon polymer having a high melting point.
 3. The invention setforth in claim 1 in which said dielectric layer is composed ofpolytetrafluoroethylene.
 4. Apparatus according to claim 2 in which saidenclosure is elongated and generally tubular in shape, said electricalinsulation means forming a part of said enclosure comprises a generallycylindrical shell of insulating material, said fixed capacitor electrodecomprises a generally tubular metallic extension of said insulationmeans having a closed end, said movable electrode is a coaxial generallytubular member fitting within said fixed electrode and said dielectriclayer is emplaced between the inside surface of said fixed electrode andthe outside surface of said movable electrode.
 5. Apparatus according toclaim 4 in which said dielectric layer is fixed to the outer peripheryof said movable electrode, thereby to form a lubricatory interface withsaid fixed electrode.
 6. Apparatus according to claim 5 including two ofsaid fixed electrodes in axial series separated by said cylindricalshell of insulating material, and in which said movable electrode isaxially positionable in differential capacitance relationship withrespect to said two fixed electrodes.
 7. The invention set forth inclaim 2 in which said fixed electrode is a substantially planar surfacenormal to the axial centerline of said enclosure, said movable electrodeis a generally disc shape member of resilient metal predisposed toassume a generally conical shape in the absence of axially appliedforce, said dielectric layer is a planar sheet emplaced between saidfixed and movable electrodes, and said mechanical control member isarranged to exert a force tending to flatten said movable electrode asits peripheral edges slide radially outward thereby to correspondinglycontrol the capacitance between said electrodes.
 8. The invention setforth in claim 5 in which said movable electrode is a generally tubularclosed-end member and said dielectric layer substantially encases saidmovable electrode, whereby the fully meshed capacitance between saidfixed and movable electrodes is proportional to the surface area of saidmovable electrode in juxtaposition with said fixed electrode, includingthe area of said closed end.
 9. A solid dielectric variable vacuumcapacitor comprising: at least one generally tubular metallic outermember forming a first capacitor plate; a generally tubular metallicinner member of outside diameter smaller than the inside diameter ofsaid outer member; a generally tubular fluorocarbon polymer dielectricmember of inner and outer diameters differing from said inner memberoutside diameter and said outer member inside diameter, respectively, bysubstantially only sliding fit allowances, said inner member beingaxially slidably supported within said outer member by said dielectricmember without additional bearing means; a conductive mechanical controlmember connected for providing axial sliding movement of said innertubular member to vary the capacitance between said inner and outermembers by varying the axial insertion of said inner member within saidouter member; an evacuated enclosure including said outer member as apart thereof, said enclosure providing evacuated space at leastenclosing the adjacent surfaces of said inner and outer tubular membersand said dielectric member, said enclosure also including an extendablemetallic bellows sealed between said mechanical member and an end ofsaid enclosure to permiT axial movement of said mechanical member withinsaid enclosure; and insulating means for providing an electricallyinsulated mounting for said mechanical control member externallyadjacent the connection of said mechanical control member and saidbellows, thereby to electrically isolate said inner and outer members,said inner member making external electrical connection through saidbellows and said mechanical member.
 10. Apparatus according to claim 9in which said fluorocarbon polymer is polytetrafluoroethylene.